Devil May Cry walkthrough/M20

The Underworld is creepy, and resembles the digestive tract of a space whale. Don't stand in one spot for two long, or you'll get attacked by the Underworld equivalent of intestinal parasites.

Walkthrough
When you start, run towards the camera and jump up into the tunnel on the wall. Run to a red filmy membrane that blocks your path and swing at it with your weapon to break it. Continue forward and you will enter a room with a Divinity Statue and a pair of Frosts. Kill the Frosts and smack the membrane at the far end of the room with your weapon. When you step through, you will be treated to a cutscene where Trish asks for your help. Before Dante can get to her, he's walled in on all sides and confronted by a familiar puddle of goo.

Boss: Nightmare
See Mission 16 and Mission 18 for strategy.

When you get Nightmare to critical health, you're interrupted by Trish. From here on, she will also attempt to attack you. Keep moving and be on the lookout for a yellow line indicating where she's going to attack. Once Nightmare is dead... It's not dead. After you drain its health, it gains more, putting itself back at critical health, and turns liquid but with its core exposed.

This final part is a pain. Nightmare's core will constantly bombard you with laser attacks while its little floating orbs pelt you with small arms fire and Trish tries to electrocute you. Pull out every trick in your arsenal to take it out, and don't be afraid to use your Untouchables, Devil Stars, and Vital Star. Air Raid and Vortex give you the most bang for your buck, but require Devil Trigger to use. Avoid using the Grenadegun, as you aren't able to dodge well when firing. The Shotgun has too short of a range to be effective, and the Nightmare Beta is useless. Sparda's Round Trip can deal some good damage and allow you to maneuver, but only use it after you've exhausted your DT gauge with Air Raid or Vortex, since the Sparda removes your DT gauge.

Once Nightmare is dead for real, watch the cutscene. The mission ends afterwards.